US7085400B1 - System and method for image based sensor calibration - Google Patents

Abstract

Apparatus and methods are disclosed for the calibration of a tracked imaging probe for use in image-guided surgical systems. The invention uses actual image data collected from an easily constructed calibration jig to provide data for the calibration algorithm. The calibration algorithm analytically develops a geometric relationship between the probe and the image so objects appearing in the collected image can be accurately described with reference to the probe. The invention can be used with either two or three dimensional image data-sets. The invention also has the ability to automatically determine the image scale factor when two dimensional data-sets are used.

Description

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention is directed generally to image guided medical systems, and more particularly, to systems and methods for utilizing data collected from imaging sensors to calibrate a tracking device.

2. Description of the Related Art

Image guided surgical techniques have been used with success in aiding physicians for performing a wide variety of delicate surgical procedures. These systems are typically used when the visualization of a surgical tool could be obscured by a patient's anatomy, or when the surgical tool is visible but the patient's anatomy could be difficult to visualize.

In order for these systems to be effective, points of the patient's anatomy appearing in the image must be accurately correlated to the instrument being tracked by the surgical navigation system. Accomplishing this correlation requires the accurate calibration of the tracking device. Ultimately, the calibration problem involves determining the position and orientation (POSE) of a set of points displayed by the image plane in the space defined by the tracking markers of the tracked instrument. It can be assumed that the structure associated with the tracking markers and image plane is a rigid body, so once the POSE is determined it remains constant. However, it is not possible to physically measure the POSE of the points in the image plane.

In addition to being robust and accurate, a preferred calibration scheme must be an uncomplicated procedure which can be performed quickly in the field by minimally trained personnel.

SUMMARY OF THE INVENTION

The present invention is directed generally to image guided medical systems, ERSON, and, particularly, to systems which correlate tracked instrument positions to image data obtained from a patient. More specifically, the present invention is directed to a device and method for registering tracking device outputs with image data.

To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention is directed to an apparatus and method for determining the calibration of a tracking device using image data collected by a tracked probe.

In one aspect of the invention, a surgical navigation system performs the tracking of a position of a calibration jig and a position of a probe. An imaging system performs the collection of a plurality of images of at least one calibration pattern contained in the calibration jig. After receiving the images from the imaging system, the navigation system locates the centers of intersection points in image space associated with each calibration pattern for each of the plurality of images, and extracts a calibration point therefrom. The navigation system then determines the three-dimensional position for at least one calibration point in probe space for each of the plurality of images. Furthermore, the navigation system then relates the positions of the calibration points in image space and the positions of the calibration points in probe space with a coordinate transform. Using this coordinate transform, the navigation system computes a reference position of the image in probe space and stores this reference position. Once the coordinate transform and image reference position in probe space are obtained, the calibration process is complete.

In another aspect of the invention, a surgical navigation system performs the tracking of a position of a calibration jig and a position of a probe. A three-dimensional imaging system performs the collection of a volumetric image of at least one calibration pattern contained in the calibration jig. After receiving the volumetric image from the imaging system, the navigation system extracts two-dimensional slices from the volumetric image and locates centers of intersection points in slice space associated with each calibration pattern for each of the plurality of slices, and extracts a calibration point therefrom. The navigation system then determines the three-dimensional position for at least one calibration point in probe space for each of the plurality of slices. Furthermore, the navigation system then relates the positions of the calibration points in slice space and the positions of the calibration points in probe space with a coordinate transform. Using this coordinate transform, the navigation system computes and stores a reference position of the volumetric image in probe space. Once the coordinate transform and volumetric image reference position in probe space are obtained, the calibration process is complete.

The invention allows for accurate, free-hand calibration of a tracked instrument which can be performed by minimally trained personnel. The calibration jig may be a sterile object and used within an operating room prior to the performance of a surgical procedure. Furthermore, problems associated with the calibration jig or the imaging system can easily be detected by inspection of the images as the calibration is being performed.

It is to be understood that both the foregoing general description and the following detailed description are exemplary only and are not intended to be restrictive of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention.

FIG. 1 is a simplified side view of an embodiment of a system for the calibration of a probe in accordance with the present invention.

FIG. 2ais a perspective view of an embodiment of a calibration jig in accordance with the present invention.

FIG. 2bdepicts a top view of the calibration jig shown inFIG. 2a.

FIG. 3ais a perspective view of an ultrasound probe imaging a jig containing a point target.

FIG. 3bis a top view of the jig shown inFIG. 3aillustrating the elevation imaging problem due to a non-ideal imaging plane.

FIG. 4 represents an image of the calibration jig inFIGS. 2a,bformed by an ultrasonic imaging system.

FIG. 5 is a top view showing how the imaging plane intersects an embodiment of a calibration pattern.

FIG. 6 illustrates an image of the calibration pattern ofFIG. 5 showing the intersection points in the image.

FIG. 7 is a perspective view of an embodiment of a calibration jig showing different planes intersecting a pattern at differing geometries, and the resulting images corresponding to each geometry.

FIG. 8 is a flow chart illustrating methods and systems in accordance with the present invention.

FIG. 9 shows a set of collected images each containing calibration points lying in a different part of the image plane and their relationship to probe space reference.

FIG. 10 is a flow chart illustrating the steps of determining a relationship between a probe using two dimensional images.

FIG. 11 illustrates an exemplary three-dimensional image of an embodiment of a calibration pattern and its corresponding two dimensional slices of the volumetric image.

FIG. 12 is a flow chart illustrating methods consistent with the steps of calibrating a probe using a three dimensional image.

FIG. 13 is a block diagram of an exemplary computer system consistent for use with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

FIG. 1 illustrates an exemplary calibration system andmethod100 which is consistent with the present invention.Stationary calibration jig110 supported byplatform115 is scanned bytechnician105 by manipulatingprobe130.Probe130 comprises a transducer which transmits signals in the direction of thejig110. Signals reflected fromjig110 can then be received byprobe130 and fed by cable toimaging system120.Imaging system120 processes the received signals and forms images which can be displayed onmonitor140. In the preferredembodiment imaging system120 is a standard ultrasonic imaging system; however, it should be appreciated that other types of imaging systems, such as microwave, X-ray, or optical could also be used with the present invention.Probe130 typically is tracked, and for example, has a plurality oftracking markers160 or other trackable features attached at its distal end. Additionally,calibration jig110 can also have a plurality oftracking markers125 attached to its surface. In the preferred embodiment,markers125 can be attached to the periphery of the upper surface ofcalibration jig110.

Tracking markers125 and160 can include by way of example only reflectors/emitters operating in the optical, infrared, electromagnetic, and/or acoustic domains and/or other suitable devices known in the art. For example, tracking markers such as those supplied by Northern Digital Incorporated may be used in conjunction with the present invention. It should be noted that thejig calibration markers125 do not have to be identical to theprobe calibration markers160.

The tracking markers can be used to localizeprobe130 andcalibration jig110. Localization is the process of determining the position and orientation of an object of interest and tracking movement of the object over some period of observation.

The manner in whichsensor170 tracks the positions ofcalibration jig110 and probe130 is well known in the art and is therefore only described generally.Sensor170 comprises adetector array175 which can be used to detect energy from the trackingmarkers125 and160. In the preferred embodiment, the array is a set of CCD cameras which sense infrared energy. However, other sensors may be used which operate at acoustic, electromagnetic, optical, radiological, and/or other frequencies. For example,sensor array175 is located and suspended by a mount in such a manner as to provide a line of sight between the mount and trackingmarkers125 and160. Signals fromsensor170 are coupled intocomputer150 which processes the received data to determine the position of the markers and, consequently the position of the object attached thereto. Based on the relative positions of the markers as sensed in detector array, the positions of objects can be determined and representations of the objects can be displayed onmonitor180. The tracking technology employed in the present invention may be the same as that used in the STEALTH STATION® Treatment Guidance Platform available from Medtronic Sofamor Danek, Inc.

Surgical navigation systems which perform localization functions to assist in medical procedures are well established. Such systems are disclosed, for example, in PCT Application No. PCT/US95/12894 (Publication No. WO 96/11624) to Bucholz, the entire disclosure of which is incorporated by reference.

Referring further toFIG. 1,imaging machine120 is coupled tocomputer150 through suitable link orconnection190.Connection190 may be, for example, a device specific digital interface or a generic video output signal. Image data from this connection may be used bycomputer150 to perform the calculations required to calibrate the tracking ofprobe130. Images taken byprobe130 have specific characteristics due to the construction of thecalibration jig110. These characteristics, which are geometric in nature and will be described in detail later, can be used to determine the position of points appearing in the image in a coordinate system referenced tocalibration jig110. The space defined by the coordinate system referenced tojig110 is termed jig space for purposes of this document. Since the surgical navigation system is tracking the calibration jig,navigation system computer150 can also determine the positions of the calibration points in a coordinate system referenced to the probe, defined as probe space for purposes of this document. Positions of these same calibration points are also measured in the image coordinate system, or image space, which typically are measured in pixels. By utilizing the positions of the same calibration points described in the both image space and probe space, a relationship between the spaces can be derived. Once this relationship is determined, any pixel in the image can be accurately described in probe space, and thus the calibration will be complete.

Furthermore, althoughFIG. 1 shows asingle computer150 performing the localization and calibration functions, multiple computers may be implemented as a single computer to perform the functions performed bycomputer150. Although a STEALTH STATION® image guided system manufactured by Medtronic Sofamor Danek has been identified, it will be appreciated that the present invention may be utilized with other types of computer systems. In addition, even thoughFIG. 1 shows only onedisplay180 coupled tocomputer150, multiple displays of various types known in the art may be coupled tocomputer150 in order to provide information to the user of the system.

FIG. 2ashows a perspective view ofcalibration jig110. In the preferred embodiment, a plurality of trackingmarkers125 can be attached around the perimeter of the jig's upper surface so the jig may be localized by the surgical navigation system. The calibration jig contains a structure which includes a calibration pattern. In general, the calibration pattern may include of a plurality of parallel members joined by a common member. The geometric relationship between the common member and the parallel members is preferably known and well controlled. The calibration pattern in its most basic form can include a set ofwires220 with a diameter comparable to the wavelength of the radiation from the imaging device. The set of wires may be arranged in a “Z” pattern as shown in theFIG. 2a. The mounting points of the two parallel wires within the “Z” pattern is preferably known in calibration jig space at least to the precision desired by the calibration process. While only one “Z” pattern is shown inFIG. 2aand in subsequent figures for purposes of clarification, it should be appreciated that a plurality of “Z” patterns may be contained withincalibration jig110. It should be understood that other suitable patterns can also be used.

For the preferred embodiment,wires220 may be constructed of nylon and are submerged in a suitablehomogenous imaging medium210 having a known value for the velocity of propagation of the ultrasound wave emitted by the probe. Such a medium, for example, may be water, ultrasound gel, or some other substance which approximates wave propagation through a biological organism. Images of the “Z” pattern are preferably collected with the imager as perpendicular to the plane formed by the “Z” pattern.

FIG. 2bis a view as shown from line A′–A of the perspective drawing ofFIG. 2a. The wires of “Z”pattern220, submerged inimaging medium210, are mounted in accurately known locations on the inner wall ofjig110.Tracking markers125 are shown in this embodiment as surrounding the upper surface ofjig110.

Performing accurate image-based calibration using small point-target like structures, such as a needle tip, can be difficult due to elevation imaging effects.FIG. 3adepicts such a situation where anultrasound probe330 is imaging apoint target320. Ideally, no image would be collected frompoint target320 untiltrue imaging plane340 intersects it. The true imaging plane has a negligible thickness and is used as the reference plane for the calibration process. In practice, elevation imaging effects can create an image of point target although the true plane of the ultrasound image may be some distance frompoint target320.FIG. 3b, taken from the perspective of line A′–A, shows this situation.True imaging plane340 is located at some distance away frompoint target320; however, due to the elevation effect,point320 lies withincollection plane350.Collection plane350 is centered aroundtrue plane340 and has some finite thickness. As shown inFIG. 4, the resultingimage470 collected fromjig300 will display arepresentation460 ofpoint target320, even thoughideal plane340 is at a distance greater than a resolution cell away frompoint target320. The elevation effect results in some level of uncertainty when attempting a precise operation such as calibration. To compensate for this uncertainty, a realistic model ultrasound imaging preferably considers the image plane having a non-negligible thickness.Calibration pattern220 compensates for the elevation imaging problem by providing a line target to produce points which can be used to calibrate the probe.

FIG. 5 shows a top view of “Z”pattern220 being intersected by both thetrue imaging plane500 and thecollection plane510 of the imaging system.True imaging plane500, represented inFIG. 5 as line B′–B, has a negligible thickness. Objects captured within this plane will have no elevation errors associated with their location in the image.Collection plane510, however, has a finite thickness and objects captured within this plane can appear as if they were imaged at thetrue imaging plane500, thus creating a position error for these objects in the output image. The “Z” jig compensates for these errors by providing a line target for the calibration points. The line targets of the “Z”pattern220 pass through the entire thickness ofcollection plane510. The resulting image will show an integration of the energy reflected by the wire as it passed through the entire elevation dimension of the collection plane.Shaded portions530a–band532, which can be interpreted as projections of the wire onto thetrue imaging plane500, represent the image of the wire in the output image.

FIG. 6 depicts an example of anoutput image600 collected from the “Z”pattern220. The image is taken from the viewing perspective B′–B shown inFIG. 5. Each oval630a–band632 inimage600 represents an intersection of the wire,530a–band532, respectively, with the entire width ofcollection plane510. For the calibration problem, the points of interest are where the wires actually intersect thetrue imaging plane500; these intersections lie at thecenters635a–band637 of each of theovals630a–band632 respectively. The vertical portions of the centers are interpreted as the centers of thewires220 in the vertical dimension ofimage600. The centers may either be selected manually by the user or automatically by the computer. One advantage of utilizing the “Z” pattern is that in producingcenters635a–band637 inimage600, the intersection of the wire withtrue imaging plane500 is accurately determined and therefore errors due to the elevation effect are avoided. In order to properly perform the calibration, the coordinates of identical points are preferably found in image space and in probe space. The image point which is used to perform the calibration iscenter637 ofmiddle oval632. These positions, taken over many different images, form a set points defined as calibration points.

FIG. 7 illustrates the technique of determining the three-dimensional coordinates of calibration points in the calibration jig coordinate system, or jig space, based upon the two outside centers and the middle center. Coordinatesystem720 is the reference used to localize points in jig space. The X-Y plane of the coordinate system is theinside wall702 ofjig110 and the Z-axis lies along the lower inside edge, as shown.First image730 is the result of placingtrue imaging plane700 close to the origin of coordinatesystem720. Due to the geometry of the Z pattern,imaging plane700 intersectsdiagonal wire707 of the Z pattern closer to the left wire atpoint706. As a result, thecenter734 lies closer to the left side ofimage730.Second image740 is the result of placingtrue imaging plane710 further from the origin of coordinatesystem720. In this instance,imaging plane710 intersects thediagonal wire707 of the “Z” pattern closer to the right wire atpoint708. The resultingimage740 shows themiddle center744 lying closer to the right side of image. These two examples shown inFIG. 7 illustrate that the lateral position of the middle center in the output image can be directly correlated to the Z coordinate in jig space of thediagonal wire707 when it intersects the true imaging plane. The X and Y coordinates of the calibration point in jig space are identical to location of the two parallel wires, which are precisely known and remain constant for any value of Z. The technique of determining the Z coordinate is preferably based on the Brown-Roberts-Wells (BRW) method used in some Computer Tomography applications Essentially it computes the ratio of the distances from one of the outside points to the center point and the remaining outside point in order to compute the location in jig space of the calibration point. This technique is well known in the art and is described in the paper “Coordinate Transformation and Calculation of the Angular Depth Parameters for a Stereotactic System,” Medical Physics, Vol. 14, No. 6, November/December 1987 by Chen B. Saw, et al., which is incorporated by reference.

Another preferable advantage of employing “Z”pattern220 over other calibration techniques is the elimination of one axis of movement necessary to position images for calibration. Motion of the imager along the long axis of the “Z” results in the middle center moving laterally in the output image. Thus an automated calibration system would preferably only need to move the imager along the long axis and perpendicular to the axis of a “Z” pattern. Since the calibration point is computed based on the image, preferably, there are no predefined positions for placing the imaging plane within the calibration jig.

Referring toFIG. 8, the processes or steps associated with calibration procedure is illustrated at800. Initially,calibration jig110 and probe130 are tracked using the surgical tracking system throughout the entire calibration procedure instep810. An initial image ofcalibration jig110 is collected (step820). From this initial image, the orientation of the intersection points is determined. This could be a automated process carried out bycomputer150. However, it is also possible for the user to determine this orientation and provide this information into the computer manually via a keyboard or through a graphical user interface (step830). Instep840, the center points of each intersection in the initial image is determined, and their pixel location is recorded as its image space coordinate. Again, this step may be performed manually by the user or could be carried out automatically bycomputer150. Instep850, the Z coordinate of the calibration point is computed in jig space by determining the ratio of the distance between centers of the two outside intersection points and the distance between the center of one outside intersection point and the center of the middle intersection point. From analysis of these ratios, the position of the calibration (center) point in jig space can be calculated. Instep860, the calibration point is transformed from jig space to probe space. This transformation is readily calculated bycomputer150 since bothjig110 and probe130 positions are known in detector space. Once coordinates of the calibration point are known in both image space and probe space, an initial estimate of the transform relating the two spaces and the scale factor between them is made. Afterwards, points associated with several hundred more images, for example, are calculated and steps840–870 are repeated on a large set of calibration points to refine the transform and scale factor which relate image space to probe space (step880). After the transform and scale factors are accurately known, the origin of the image in probe space coordinates is defined which completes the calibration procedure (step890).

The goal of the calibration process is to be able to relate points described by pixel locations in image space to positions described in three-dimensional probe space. In order to accomplish this, a mathematical transform between the two spaces, or coordinate systems, is preferably determined.FIG. 9 depicts an exemplary three-dimensional coordinate system,920, which is referenced to probe130 and used to describe points in probe space. Coordinatesystem930 is a two dimensional coordinate system which is used to describe the location of points within the images. During the calibration process, a set ofimages910 is collected. Each image within the set representscalibration pattern220 as sensed byprobe130 as the probe is moved along the calibration jig's longitudinal dimension (Z axis of coordinatesystem720 shown inFIG. 7). The number of images, N, within the set can vary, but typically several hundred are used to perform the calibration. Each of the images withinset910 contains at least one calibration point; however, for purposes of clarity, only one calibration point per image is shown inFIG. 9. As described earlier, the centers of calibration points900a–nare identified in each image, resulting in at least several hundred image points whose locations in jig space can be calculated. Once this is accomplished, calibration points900a–ncan readily be transformed to probe space referenced by coordinatesystem920. The final stage in the calibration process is to utilize thepoints900a–ndescribed in both image space and probe space to derive a coordinate transform and scale factor between the two spaces. The computation of this transform is described in detail below.

A technique to derive a transform between two coordinate systems given a set of identical points described in each system is provided in “Closed-form Solution of Absolute Orientation using Unit Quaternions,” Journal of the Optical Society of America, Vol. 4, No. 4, April 1987 by Horn, which is incorporated by reference.FIG. 10 presents a brief description of thismethod1000 as it applies to the present invention. Initially, identical points described in both image space and probe space are collected instep1010. Image space points are then rotated so that they align with points given in probe space (step1020). After rotation, the centroids of all the points in both probe and image spaces are computed. These values are then used to find the translation between the two systems (step1030). The image space points are then translated in order to align with probe space points (step1040). Finally, instep1050, the scale of the image space points is adjusted to minimize the point-to-point error with the probe space points.

Referring toFIG. 11, another embodiment of the invention allows the calibration of image volumes produced by three-dimensional imaging machines, as well as reconstructed volumes from two-dimensional imaging machines. Instead of an image by image collection of intersection points for the calibration, the entire volume is processed1100. By collecting slices of voxels, or volume element which is the 3-D counterpart to “pixels” associated with the “Z”pattern1120, thecorresponding intersection points1130 can be computed. Then, by applying the same techniques as previously disclosed herein for the two dimensional calibration, a calibration transform for the volume is computed. The preferred embodiment for a three-dimensional sensor is an ultrasound device; however, other methods of producing three dimensional imagery could also be used within the scope of the present invention.

Referring toFIG. 12, steps consistent with the present invention for calibrating three-dimensional images are shown (1200). Initially, thecalibration jig110 and probe130 are tracked with a surgical navigation system (step1210). A three-dimensional volumetric image is collected and processed with an imaging system (step1220). A first slice is extracted from the volumetric image and the orientation of the points (1130) in the slice is determined (steps1230 and1240).Steps1230 and1240 may be performed by the operator. However, they may also be performed automatically by the computer. Instep1250, the centers of the intersections of the “Z” pattern are determined for the first slice. Afterwards, the coordinates for the middle intersection point, or the calibration point, is determined in jig space using the BRW method described earlier (step1260). Instep1270, an initial transform is computed which relates probe space and image space. Unlike the two-dimensional case, the scale factor is not computed since these factors are already known. After an initial estimate for the transform is made, the computer will extract several hundred more slices (step1280), for example, and repeatsteps1250–1270 in order to refine the initial estimate of the transform. After the transform is determined, the origin of the volumetric image is described in probe space, completing the calibration procedure (step1290).

Referring toFIG. 13, components and modules of acomputer system150 used to perform various processes of the present invention are described. Although a STEALTH STATION® image guided system manufactured by Medtronic Sofamor Danek has been identified, it will be appreciated that the present invention may be utilized with other types of computer systems. One aspect of thecomputer system150 includes a graphical user interface system operating in conjunction with a display screen of adisplay monitor180. The graphical user interface system is preferably implemented in conjunction withoperating system1315 runningcomputer150 for displaying and managing the display objects of the system. The graphical user interface is implemented as part of thecomputer system150 to receive input data and commands from aconventional keyboard1320 andmouse1325. For simplicity of the drawings and explanation, many components of a conventional computer system have not been illustrated such as address buffers, memory buffers, and other standard control circuits because these elements are well known in the art and a detailed description thereof is not necessary for understanding the present invention.

A computer program used to implement the various steps of the present invention is generally located inmemory unit1300, and the processes of the present invention are carried out through the use of a central processing unit (CPU)1305. Those skilled in the art will appreciate that thememory unit1300 is representative of both read-only memory and random access memory. The memory unit also contains adatabase1350 that stores data, for example, image data and tables, including information regarding the probe, and geometric transform parameters, used in conjunction with the present invention.CPU1305, in combination with the computer software comprisingoperating system1315,scanning software module1330,tracking software module1335,calibration software module1340, anddisplay software module1345, controls the operations and processes ofcomputer system150. The processes implemented byCPU1305 may be communicated as electrical signals alongbus1360 to an I/O interface1370 and avideo interface1375.

Scanning software module1330 performs the processes associated with creating a coordinate reference system and reference images for use in connection with the present invention and are known to those skilled in the art.Tracking software module1335 performs the processes for tracking objects in an image guided system as described herein and are known to those skilled in the art.Calibration software module1340 computes the coordinates of the calibration points in jig space and implements method to determine the transform between image space and probe space.

Display software module1345 formats the image data for display onmonitor180 and can identify the center positions of the intersection points in displayed images with icons. Typically these icon are represented as cross-hairs. The display software module also works in conjunction with the graphical user interface and allows the user to determine the orientation of the initial image.

Image data1355 can be fed directly intocomputer150 as a video signal throughvideo interface1375. Alternatively, the data could also be supplied digitally through I/O interface1370. In addition, items shown as stored in memory can also be stored, at least partially, onhard disk1380 if memory resources are limited. Furthermore, while not explicitly shown, image data may also be supplied over a network, through a mass storage device such as a hard drive, optical disks, tape drives, or any other type of data transfer and storage devices which are known in the art.

The foregoing description is present for purposes of illustration and explanation. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications of variations are possible in light of the above teachings or may be acquired from practice of the invention. The principles of the invention and its practical application enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.

Claims (19)

What is claimed is:

1. A method of calibrating a tracking device to an image, comprising:

tracking a position of a calibration jig and a position of a probe;

collecting a plurality of images of at least one calibration pattern of the calibration jig with the probe;

locating centers of intersection points in image space associated with each calibration pattern for each of the plurality of images and extracting a calibration point therefrom;

determining the position for at least one calibration point in probe space for each of the plurality of images;

relating the positions of the calibration points in image space and the positions of said calibration points in probe space;

computing an image space reference position in probe space; and

storing the image space reference position.

2. The method ofclaim 1 further including:

identifying an orientation of the intersection points in a first image of the plurality of images collected;

computing a position component of the calibration point in calibration jig space by comparing the distances from the center of one of the intersection points near each image edge to the centers of two other intersection points for each image; and

transforming the positions of calibration points described in calibration jig space to positions described in probe space.

3. The method ofclaim 2 wherein the identification of orientation is performed automatically, specified manually, or combinations thereof.

4. The method ofclaim 2 further including:

receiving positions of identical calibration points described in image space and described in probe space;

rotating the calibration points described in image space to align with the calibration points described in probe space;

computing centroids of the calibration points described in the rotated image space and the calibration points described in the probe space;

translating the calibration points described in the rotated image space to the calibration points described in probe space; and

adjusting the scale of the calibration points described in the rotated and translated image space to minimize the point to point error with the calibration points described in probe space.

5. The method ofclaim 1 wherein the plurality of images are collected utilizing a probe comprising an ultrasonic transducer generating and receiving ultrasonic signals and further utilizing a processing system which forms two dimensional images from the ultrasonic signals received by the ultrasonic transducer.

6. The method ofclaim 1, wherein collecting a plurality of images of at least one calibration pattern includes positioning an ultrasound probe relative to the calibration jig to image the calibration jig with the ultrasound probe;

directing ultrasound waves with the ultrasound probe into a jig space of the calibration jig.

7. A method of calibrating a tracking device to a three-dimensional volumetric image, comprising:

tracking a position of a calibration jig and a position of a probe;

collecting a volumetric image of at least one calibration pattern contained in the calibration jig;

extracting two-dimensional slices from the volumetric image;

locating centers of intersection points in slice space associated with each calibration pattern for each of the plurality of slices and extracting a calibration point therefrom;

determining the position for at least one calibration point in probe space for each of the plurality of slices;

relating the positions of the calibration points in slice space and the positions of said calibration points in probe space;

computing a reference position of the volumetric image in probe space; and

storing the reference position of the volumetric image.

8. The method ofclaim 7 further including:

extracting a first slice from the three-dimensional image;

identifying an orientation of the intersection points in the first slice;

computing a position component of the calibration point in calibration jig space by comparing the distances from the center of one of the intersection points near each slice edge to the centers of two other intersection points for each slice; and

transforming the positions of calibration points described in calibration jig space to positions described in probe space.

9. The method ofclaim 7 further including:

receiving positions of identical calibration points described in slice space and described in probe space;

rotating the calibration points described in slice space to align with the calibration points described in probe space;

computing centroids of the calibration points described in the rotated slice space and the calibration points described in the probe space; and

translating the calibration points described in the rotated slice space to the calibration points described in probe space.

10. The method ofclaim 7 wherein the three-dimensional volumetric image is collected utilizing a probe which is an ultrasonic transducer which generates and receives ultrasonic signals and a processing system which forms the three-dimensional image from the ultrasonic signals received by the ultrasonic transducer.

11. A system for registering a tracking device to an image, comprising:

a means for tracking a three-dimensional position of a calibration jig and a position of a probe;

a means for collecting a plurality of images of at least one calibration pattern on the calibration jig with the probe;

a means for providing the plurality of images to the tracking means;

a means for locating centers of intersection points in image space associated with each calibration pattern for each of the plurality of images and extracting a calibration point therefrom;

a means for determining the position for at least one calibration point in probe space for each of the plurality of images;

a means for relating the positions of the calibration points in image space and the positions of said calibration points in probe space;

a means for computing an image space reference position in probe space; and

a means for storing the image space reference position.

12. The system ofclaim 11 further including:

a means for identifying an orientation of the intersection points in a first image of the plurality of images collected;

a means for computing a position component of the calibration point in calibration jig space by comparing the distances from the center of one of the intersection points near each image edge to the centers of two other intersection points for each image; and

a means for transforming the positions of calibration points described in calibration jig space to positions described in probe space.

13. The system ofclaim 12 wherein the identification means is performed automatically, specified manually, or combinations thereof.

14. The system ofclaim 12 further including:

a means for receiving positions of identical calibration points described in image space and described in probe space;

a means for rotating the calibration points described in image space to align with the calibration points described in probe space;

a means for computing centroids of the calibration points described in the rotated image space and the calibration points described in the probe space;

a means for translating the calibration points described in the rotated image space to the calibration points described in probe space; and

a means for adjusting the scale of the calibration points described in the rotated and translated image space to minimize the point to point error with the calibration points described in probe space.

15. The system ofclaim 11 wherein the means for collecting images further comprises:

an ultrasonic transducer means for generating and receiving ultrasonic signals; and

a processing means for forming two dimensional images from the ultrasonic signals received by the transducer means.

16. A system for calibrating a tracking device to a three-dimensional volumetric image, comprising:

a means for tracking a position of a calibration jig and a position of a probe;

a means for collecting a volumetric image of at least one calibration pattern contained in the calibration jig, coupled to the tracking means;

a means for extracting two-dimensional slices from the volumetric image;

a means for locating centers of intersection points in slice space associated with each calibration pattern for each of the plurality of slices and extracting a calibration point therefrom;

a means for determining the position for at least one calibration point in probe space for each of the plurality of slices;

a means for relating the positions of the calibration points in slice space and the positions of said calibration points in probe space;

a means for computing a reference position of the volumetric image in probe space; and

a means for storing the reference position of the volumetric image.

17. The system ofclaim 16 further including:

a means for extracting a first slice from the three-dimensional image;

a means for identifying an orientation of the intersection points in the first slice;

a means for computing a position component of the calibration point in calibration jig space by comparing the distances from the center of one of the intersection points near each slice edge to the centers of two other intersection points for each slice; and

a means for transforming the positions of calibration points described in calibration jig space to positions described in probe space.

18. The system ofclaim 16 further including:

a means for receiving positions of identical calibration points described in slice space and described in probe space;

a means for rotating the calibration points described in slice space to align with the calibration points described in probe space;

a means for computing centroids of the calibration points described in the rotated slice space and the calibration points described in probe space; and

a means for translating the calibration points described in the rotated slice space to the calibration points described in probe space.

19. The system ofclaim 16 wherein the means for collecting images further comprises:

an ultrasonic transducer means for generating and receiving ultrasonic signals; and

a processing means for forming three dimensional images from the ultrasonic signals received by the transducer means.

Images